1. Field of the Invention
The present invention relates to evaporative chillers and evaporative cooling systems, and more particularly, to an evaporative water chiller or evaporative fluid cooling system that is operable to lower the temperature of water or other liquid exiting the chiller or chiller system to below the ambient wet-bulb temperature. This may be achieved by pre-cooling the incoming air flow or airstream to a temperature below the ambient air temperature such as by using the outgoing or exiting air flow or airstream.
2. Relevant Background
Today, a large fraction of the electrical energy used in the United States and elsewhere in the world is used for cooling interior spaces, such as habitated areas of residential and commercial buildings, to desired or acceptable temperatures. In some geographic regions, cooling costs may be more than half of the annual energy cost for businesses and home owners. The electrical energy used for space cooling is not only costly but causes problems because it is concentrated into certain times of the day when highest temperatures are experienced, and this high demand can create high peaks in power demand that are difficult for power companies to satisfy. Hence, there is an ongoing need for reducing the amount of energy needed for cooling and for better distributing the demand to reduce the size of spikes or peaks in demand. Reducing demand for electricity is a vital and growing concern as the human population increases, as more and more countries become industrialized and more urban, as concerns heighten over global warming from fossil fuel combustion, and as the availability of fossil fuels dwindles and the associated prices rise. One way to control electricity or power consumption is to develop lower-energy, alternative cooling systems that have the potential to reduce overall and peak electricity usage.
However, it has proven difficult to design cooling systems and devices that can effectively compete with refrigerant-based air conditioning (A/C) systems to significantly reduce overall power consumption. Evaporative coolers are one approach, but a number of disadvantages have blocked widespread use of these cooling systems. Evaporative cooling involves evaporation of a liquid to cool an object or a liquid in contact with an airstream. When considering water evaporating into air, the web bulb temperature of the ambient air (as compared with the dry bulb temperature) is a standard measure for the potential for evaporative cooling systems, and the greater the difference between the wet bulb and dry bulb temperatures the greater the possible evaporative cooling effect. Evaporative cooling is a fairly common form of cooling for buildings for thermal comfort since it is relatively cheap and requires less energy than many other forms of cooling. However, evaporative cooling requires a water source as an evaporative and is presently only efficient when the relative humidity is low, which has restricted its use to geographic regions with dry climates.
Smaller scale evaporative coolers are often called swamp coolers, and the typical swamp cooler passes an air stream from outside of the building or interior space through the swamp cooler to contact water or other liquid in the cooler. The air is cooled by evaporation of the water, and the cooled air is directed by fans into the building or interior space. Traditional evaporative or swamp coolers have met with a fair degree of market acceptance because they work well in arid and semi-arid regions and are inexpensive to purchase and operate. While such coolers can often provide most or all of the cooling needed for a home or business, they suffer from several disadvantages. Swamp coolers are generally incompatible for integration with compressor-based A/C because they are “pass-through” systems in which conditioned air must be allowed to flow out of the building. They also require large air flow rates, and may be noisy. Further, evaporative coolers in which the cooling air contacts the water may introduce mold and allergens into the interior of the building and often unacceptably raise the indoor humidity making it “muggy” in the building. Evaporative coolers also can require significant maintenance and often require winterization to avoid damage.
An alternative cooling system involves the use of an evaporative cooling system that functions by cooling a volume of liquid such as water by evaporating a portion of the cooling liquid in a stream of ambient or outdoor air. Such systems are referred to herein as “evaporative chillers.” The cooled or chilled liquid is then circulated through piping of an air-to-water heat exchanger to cool the air blown or drawn through the exchanger. The air is cooled as heat is transferred to the water in the pipes and does not directly contact the water. The cooler air is returned to the interior spaces of the building. Evaporative chillers, which are also known as cooling towers, are more common in commercial buildings and can provide a large portion of the required cooling. Evaporative chillers are sometimes unable to lower the temperature of the interior space or building to an acceptable level, and in these cases, conventional compressor and refrigerant based air conditioning may be used to supplement the cooling achieved by evaporative cooling (however, this reduces the energy savings provided by use of the evaporative cooling system). When compared with swamp coolers and similar systems, evaporative chiller systems are compatible with compressor-based A/C units, do not introduce allergens or humidity to the cooling air (because there is no direct contact between indoor air and the chilled water), and do not require large air flow through the interior spaces of the building. In addition, evaporative chillers integrate well with typical HVAC practices in that the location of the chiller unit is flexible and existing ductwork can be utilized. Evaporative chillers are also compatible with radiant cooling technologies that are gaining acceptance in some areas.
Even in light of these advantages, evaporative chillers have not been widely used for cooling in the residential market. There are at least two main reasons that evaporative chillers are not attractive to home owners and residential builders. Depending on the wet bulb temperature, evaporative chillers often will not be able to cool the flowing coolant or water to a low enough temperature to effectively cool a building or interior space. Evaporative chillers may be seen as an unnecessary expense or an expense that will require many years to recoup based on potential energy savings. The costs associated with an evaporative chiller may be particularly unpalatable if a backup A/C system is still required to handle higher loads or to cool on hotter days. Thus, the ability of an evaporative chiller to more effectively lower the water temperature relative to the ambient wet bulb temperature is key to the success of such cooling systems.
Two-stage evaporative coolers have also been developed. These units combine both an indirect and a direct evaporative cooling stage to generate air that can sometimes reach the ambient wet bulb temperature or below. This is achieved by lowering the wet bulb temperature of the ambient air by first passing it through the indirect stage (in which no humidity is added). The second, direct evaporative cooling stage adds humidity and further reduces the air temperature. Lower air temperatures mean that these coolers can provide the required level of cooling under a wider range of ambient conditions and geographical areas than typical swamp coolers. Unfortunately, because these two-stage coolers still produce chilled air (rather than water) many of the disadvantages of swamp coolers still apply. This, along with the complexity and cost of two-stage coolers, has prevented their rapid market penetration. In theory, the two-stage designs could be adapted to produce water rather than air (by utilizing the water in the sump). Besides the cost and complexity factors, this approach has at least two disadvantages compared to the system described in the present invention. First, the two-stage cooler used as a water chiller will exhaust cooled air to the ambient environment, thus wasting cooling power and increasing water consumption. Second, the theoretical temperature to which water can be cooled is not as low as in the present invention. That is because the first (indirect) stage can only reduce the air temperature to the ambient wet bulb temperature (and typically higher in practice), whereas in the present invention the use of cooled exhaust air to cool the incoming air makes possible pre-cooling air to below the wet bulb temperature. Others have described chiller designs based on recycling exhaust air from the chiller to pre-cool incoming air. However, those designs do not take advantage of the “gradient chilling” concept of the present invention (explained below) that allows further reduction of water temperatures.
Hence, there remains a need for cooling systems that are more energy efficient and preferably that more effectively implement evaporative cooling to cool buildings or interior spaces within a residence or commercial structure. Preferably, such cooling systems would include an evaporative chiller that is designed to provide improved levels of cooling with low energy consumption and that would be adapted for use in the residential or housing market as well as in commercial settings.